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The idea that birds are descended from dinosaurs has once again been defended following attacks on the theory earlier this year.

Writing in this week's journal Nature, two Canadian researchers have reinterpreted Triassic fossil evidence of the reptile Longisquama insignis that was recently interpreted to support the claim that feathers did not evolve in dinosaurs.

"We believe that the dorsal appendages of Longisquama are highly modified scales, as suggested previously, rather than feathers," write biologist Robert R. Reisz from the University of Toronto and palaeobiologist Hans-Dieter Sues from the Royal Ontario Museum.

The fossils in question are part of a touring Russian collection.

In a June issue of Science, zoologist John Ruben and palaeontologist Terry Jones at Oregon State University, argued that the fossils showed feathers which predated most birdlike dinosaurs and that Longisquama might even be the distant evolutionary ancestor of birds.

Prior to this claim, the oldest animal that everyone agreed was a feathered bird was called Archaeopteryx, which first appeared in the fossil record about 75 million years after Longisquama.

Ruben and Jones said that the broader fossil record, which showed birds flying before the evolution of the first bird-like dinosaurs, also supported their argument.

However Reisz and Sues argue that while the fossil shows a structure with a central vein, there are no branches on either side as in a feather. Rather, this fossil shows a membrane attached to each side of the central vein. They say the membranes are corrugated which gives the appearance of feathers.

"We conclude that Archaeopteryx remains the oldest known feathered tetrapod," they write.